1. Field of the Invention
This invention relates generally to the art of fabricating shaped parts by cutting a workpiece. The invention relates more particularly to a novel method of fabricating multiple parts, especially multiple metal parts which are later joined to form an assembly, from a single workpiece comprising only a fraction of material otherwise required, by cutting the workpiece along two mutually perpendicular axes, preferably with an electrical discharge machining (EDM) wire, and then removing the recast finish surfaces resulting from the electrical discharge machining operation, as by electropolishing to preclude the later origination of cracks at such surfaces during the service life of such parts as aircraft components.
2. Discussion of the Prior Art
It will become evident as the description proceeds that the fabricating method of the invention may be utilized to make a variety of parts. The invention is particularly suited for fabricating a three-piece, triple redundant jet engine mounting link for aircraft, however, and will be described in this context.
The aircraft link referred to is designed to serve as one of several mounting attachments for securing an aircraft jet engine to the engine supporting structure. The link is actually a three piece link assembly composed of three separate steel link members mutually joined in assembled relation in such a way as to provide a triple redundant load bearing support for a jet engine. This means that the link, or more correctly link assembly, is capable of safely performing its engine supporting function even though one or even two of its link members fail(s).
This engine support link or link assembly includes a center link member and two outer link members at opposite sides of the center member. The center link member is generally wishbone-shaped and has a pair of generally spaced parallel arms at one end and a single arm at the other end disposed in a plane passing midway between and parallel to the spaced arms of the member. This center link member has outer, essentially stepped side surfaces which extend in generally parallel relation along opposite sides of the single arm of the link member, then diverge outwardly away from one another at the juncture of the single arm and the parallel arms, and then extend, again in substantially parallel relation, along the outer sides of the parallel arms. Each outer link member has a generally stepped configuration and a stepped inner side surface which substantially complements and is disposed in mating face to face contact with the adjacent outer side surface of the center link member. In the ends of the link members are aligned bores to receive bolts, pins or the like for securing the link assembly to a jet engine and its supporting structure.
According to existing fabrication techniques, such a link assembly is fabricated by machining or otherwise forming its three link members from separate metal workpieces. These existing fabrication techniques have many disadvantages among the most serious of which are excessive material waste, excessive fabrication time, and relatively high fabrication cost.
One of the well known machining techniques which have been utilized in the fabrication of such link members as well as an endless variety of other parts is electrical discharge machining (EDM). EDM is well understood in the metal working art and hence need not be explained in elaborate detail in this disclosure. Suffice it to say the fabrication of a metal part by EDM involves the use of an electrically conductive machining implement of some type and a metal workpiece from which the metal part is to be fabricated. A high EDM voltage is impressed between the implement and the workpiece such that relative movement of the implement and workpiece against one another is effective to melt or vaporize the metal in contact with the implement and thus permit the implement to pass or cut through the workpiece. The workpiece may be cut to any desired shape by effecting relative movement of the EDM cutting implement along an appropriate cutting path with respect to the workpiece while the EDM voltage is impressed between them. High intensity lasers can also be used for this purpose.
One type of EDM cutting implement is a wire which has a continuously moving portion which extends between spaced guides and which engages the workpiece. A workpiece is cut by impressing a voltage between the wire and workpiece while relatively moving the wire and workpiece in such a way that the portion of the wire between the collets presses laterally against and moves along a desired path through the workpiece. If an inside cut is to be made in a workpiece, that is, a cut which does not open through the outer perimeter of the workpiece, a hole is first drilled through the workpiece at an appropriate starting position for the cut, and the EDM wire is inserted through the hole.
The surfaces of cuts made in a workpiece by an EDM wire, laser, or in any other way are referred to herein as "cut surfaces". Cuts made by an EDM cutting wire or laser have cut surfaces which are referred to in the art as "recast surfaces". These recast surfaces are actually relatively thin recast surface layers and have a relatively rough texture similar to sandpaper. When necessary, such recast surfaces are commonly machined to the desired smoothness, in order to insure against the propagation of cracks which can be initiated at a rough surface during service life. Rough surfaces or edges are therefore not acceptable in such applications as aircraft components.